1. Field of the Invention
The present invention relates to a deposited film forming method and apparatus using a high frequency discharge, and more particularly to a plasma processing method and apparatus for a plasma CVD apparatus, a plasma etching apparatus, or the like used for producing a silicon-based amorphous or microcrystalline semiconductor such as a solar cell, a photovoltaic element, a display, etc.
2. Related Background Art
Hitherto, in a deposited film forming method of forming a functional thin film using a plasma, a high frequency plasma of a frequency from 13.56 MHz to the VHF band or further to the microwave band has normally been used to realize a semiconductor element, a photovoltaic element, etc. using a non-crystalline thin film or a crystalline thin film.
For example, in a plasma CVD method using a high frequency plasma, a non-crystalline thin film or a crystalline thin film of a silicon-based semiconductor element such as a solar cell, etc. has been formed.
In order to realize a high-quality non-crystalline or crystalline thin film, it is important to generate only radicals required to obtain a high-quality thin film and cause a surface reaction. To attain this, the supply amount of a high frequency power is reduced to control generation of those radicals which are not necessary for forming a high-quality thin film.
However, such a method is not sufficient in productivity, and is not suitable for industrial use. Therefore, an effort has been made to increase the deposition rate. However, in order to attain this, a large volume of a source gas needs to be decomposed, so that it is effective to increase the supply amount of a high frequency power and to raise the frequency of the high frequency power to increase the density of the plasma.
However, increasing the high frequency power will result in formation of not only radicals required for forming a good quality thin film but also radicals or ions which degrade the film quality. Furthermore, active radicals such as SiH2 are liable to grow in a cluster-like shape, and a plasma containing a large amount of active radicals such as SiH2, etc. will incorporate the SiH2 or clusters into the semiconductor thin film to providing a defective film, so that a good semiconductor thin film can not be obtained.
For example, when the inventors formed a non-crystalline silicon film on a 500 mm×850 mm substrate (also functioning as a grounding electrode) at a film deposition rate of 15 Å/sec by using a plasma CVD apparatus having a 500 mm×850 mm parallel plate type cathode and supplying a high frequency power of 13.56 MHz, the content of SiH2 in the thin film reaches about 10%, thereby failing to obtain a best thin film. On the contrary, there was posed the problem that a powdery substance (polysilane powder, etc.) was generated as a by-product in an end portion of the discharge space in the direction of flow of the source gas.
According to the heretofore obtained reports, there has been tried a method of applying a high frequency power in a pulse fashion to suppress the growth of active radicals to clusters, thereby preventing the generation of a powdery substance, or the like.
For example, Japanese Patent Application Laid-Open Nos. 5-226681, 5-51753, and 8-20874, etc. describe a film formation dust reducing method and a flake reducing method by applying an amplitude modulation to a high frequency power. However, little is described about the characteristic data of the quality of the formed films, and the deposition rates are not clearly described.
The inventors have applied a high frequency power with a low frequency pulse modulation to a conventional parallel plate type electrode and studied the results. However, even when the pulse modulation frequency and the duty were varied, the content of SiH2 in the formed semiconductor thin films were hardly reduced, and no thin films with good film characteristic such as photo/dark conductivities, etc. could be obtained. Furthermore, the deposition rates of the films were as small as about 3 Å/sec. The reason for this is that the discharge starting voltage applied at the rising of the modulation pulse becomes high, whereby the gas is decomposed into various kinds of radicals and ions, and only those radicals which are necessary for forming a high-quality thin film cannot be selectively generated in a large amount.
Thus, according to the conventional methods, since the defect density of a deposited film becomes high due to gas polymerization or ion bombardment and it is difficult to obtain good film quality, it has been considered general to adopt a film forming rate of 3 Å/sec or less in the light of productivity.
However, in order to improve the productivity of large-area products such as displays or solar cells using semiconductors of silicon-based non-crystalline or crystalline thin films, etc., it is necessary to form films of semiconductors such as silicon-based non-crystalline or crystalline thin films, etc. with higher quality than before with a large area at a high speed. There are similar problems with the spattering apparatus and the etching apparatus.